1. Field of the Invention
The present invention relates to a hydraulic control apparatus for a hydraulically operated vehicular transmission having hydraulic engaging elements.
2. Description of Related Art
Conventional hydraulic control apparatus of this kind is arranged in the following manner. Namely, pressurized oil from a hydraulic oil pressure source is supplied, via a manual valve, to shift valves after adjusting its pressure by a regulator valve to a certain constant pressure so as to selectively supply it to each of the hydraulic engaging elements such as clutches, brakes, or the like. The hydraulic engaging elements are thus caused to be engaged to establish a desired transmission train. At the time of disengaging the hydraulic engaging elements, each of working oil passages which are in communication with the hydraulic engaging elements is connected to an oil discharge passage via the shift valves so as to discharge the hydraulic oil in the hydraulic engaging elements through the oil discharge passages.
Further, there has recently been proposed the following, for example, in Japanese Published Unexamined Patent Application No. 224553/1989. Namely, in order to alleviate the speed shifting shocks by directly controlling or indirectly controlling (i.e, controlling the back pressure of an accumulator), in an adequate manner, the hydraulic oil pressure in the transient period of engagement of the hydraulic engaging elements, the hydraulic oil pressure in the transient period of the hydraulic engaging elements is feed-back controlled or feed-forward controlled by solenoid proportional valves or the like depending on the operating parameters such as engine load, revolution speed of the engine, revolution speed of rotating elements of the transmission, or the like.
In the above-described conventional apparatus, there are the following disadvantages. Namely, at the time of disengaging the hydraulic engaging elements, the working oil passages which are communicated with the hydraulic engaging elements are opened to the atmosphere through the oil discharge passages. As a result, they will become a condition in which they are empty of the hydraulic oil. When the hydraulic engaging elements that are once disengaged are to be engaged again, the empty working oil passages must first be filled with the working oil before the hydraulic oil pressure in the hydraulic engaging elements can be increased. This will cause a time lag, with the result that a long time is required before the hydraulic engaging elements are brought back to a condition of being engaged. Similarly, in case the transient hydraulic oil pressure in the hydraulic engaging elements is controlled, as described above, by means of solenoid proportional valves or the like, the most appropriate control becomes difficult due to the above-described time lag. Especially, since the degree of emptiness of the working oil passages normally varies in each hydraulic engaging element, the time lag fluctuates in each hydraulic engaging element. This makes an adequate control of the hydraulic engaging elements at the transient period of engagement thereof more difficult, resulting in a lower control accuracy.